This invention relates generally to stage devices for precision movement and location, such as used in lithography systems, and more particularly, to a fluid bearing with seal operable within a vacuum system.
The precise positioning of an object is required in many applications, including lithography processing in semiconductor manufacturing for forming integrated circuits on semiconductor wafers. As the circuit density of integrated circuits increases and feature size decreases, the accuracy in the methods for laying down the circuits on the semiconductor wafer must improve. Various systems and methods have been developed to attempt to improve positioning and movement of a semiconductor wafer in the lithography process. One way to increase the accuracy is to reduce system complexity and size of the stage device, thus providing greater stability of motion during positioning of the wafer.
Air bearing systems are often used to provide smooth and accurate movement between a stage and another planar surface or a guide structure. An example of a stage device for use in semiconductor processing equipment is disclosed in U.S. Pat. No. 5,760,564. The stage assembly includes two guide rails, one movable in the X direction and the other movable in the Y direction. A plurality of air bearings are attached to the guide rails and stage for movement of the guide rails and stage relative to the base. Since the bearings are attached to the stage and travel with the stage, the base must be at least as large as the diameter of the bearing plus the entire stroke (travel) of the stage. This results in a large base and stage.
Examples of air bearing systems include differentially pumped air bearing systems such as those disclosed in U.S. Pat. No. 4,191,385 to Fox and U.S. Pat. No. 4,425,508 to Lewis et al. Lewis et al. disclose an air bearing intended for use in a vacuum chamber, such as in an electron beam lithography system. The air bearing includes an air bearing plate defining a plurality of H-shaped grooves around its outer periphery and a metering valve disposed in each H-groove for introducing pressurized air into each H-groove. The pressurized air provides an air cushion between the face of the air bearing plate and the opposing face of a moving plate. The air bearing plate also includes a central vacuum region circumscribed by two concentric pump-out slots. The pressure in the vacuum region is maintained by conventional vacuum pumps. The two concentric pump-out slots are radially inward of the H-grooves to scavenge the air escaping inwardly from the H-grooves in order to prevent the air from reaching the vacuum region. Thus, the pump-out slots separate the air bearing from the central vacuum region.
Air from the air bearing flows through the small gap between the air bearing plate and the opposing face of the moving plate. Small values for the gap are required in an attempt to reduce air flow to the vacuum region. However, the smaller the gap, the tighter the necessary mechanical tolerance on the air bearing plate and the opposing surface of the moving plate, substantially increasing the manufacturing costs. A typical air bearing gap is approximately 5 microns, requiring precision machining over a relatively large surface area to substantially increase manufacturing costs, particularly because two such large precision machined surfaces are needed. In addition, because the stiffness of the air bearing is a function of this gap, adjusting the gap purely to control the air flow is often impractical.
Thus, it is desirable to provide an air bearing which is effective in preventing gas leakage into the vacuum environment and which is suitable for use within a vacuum environment and not just surrounding a vacuum. It is further desirable to provide such an air bearing which is cost effective, simple to manufacture, robust, and which produce very little wear or vibration during operation.
The air or other fluid bearing of the present invention suitable for use in a vacuum region comprises a bearing structure defining pump-out slots or passageways circumscribing a central fluid bearing outlet. One or more bearing seals are disposed along a periphery of the bearing to form a sealing wall. The sealing wall confines the fluid which escaped outside of the pump-out slots to within the perimeter of the wall formed by the sealing sheet and prevent the escape of fluid into the surrounding vacuum region. The bearing seal comprises a bridge structure including a first base, a second base, a transverse member fixed at one end to the first base and movably disposed in a slot defined by the second base, and a sealing sheet extending from the transverse member between the first and second bases.
The sealing sheets may but need not contact the opposing bearing surface. Even if there is a gap between the adjacent sealing sheets and between the sealing sheets and the opposing bearing surface, the sealing sheet structure serves to prevent nearly all of the gas molecules from escaping into the surrounding vacuum region and the gas molecules are eventually evacuated through one of the pump-out slots. The sealing wall structure may be fabricated using semiconductor type processing technology from, for example, silicon or thin metal films to providing a light flexible structure.
The seal further includes an adjustable spring force element, such as a spring, a constant force spring, or a flexural coupling, which exerts a force on the sealing sheet to ensure marginal contact with a bearing support surface to form a low-frictional seal.
The sealing sheet wall of the inventive bearing exerts a relatively low frictional force on a surface of a stage movable relative to the bearing so as to not impede the motion of the stage. The low frictional force is a result of both the relatively low mass of sealing sheet and the gap, if any, between the sealing sheets and the opposing bearing surface. Thus, the bearing of the present invention provides fast and precise movement and positioning while maintaining the vacuum surrounding the air bearing. In addition, a plurality of individual air bearings of the present invention may be utilized within a vacuum region.
The air pressure at the outer pump-out slots is preferably sufficiently low such that the air is in the molecular regime in that the molecular mean free path is much greater than mechanical system dimensions. Accordingly, the sealing sheets may be very light as the forces exerted by the impinging gas molecules against the sealing sheet structure are negligible.
The seal allows for movement of the air bearing relative to the bearing support surface and is suitable for use with an XY wafer or reticle stage. The air bearing and its seal may be adapted as a journal shaft-type bearing.
The bearing of the present invention is most appropriately applicable to gas bearings. The bearing of the present invention can be advantageously applied to fluid bearings if the bearing is designed to provide successful scavenging of the fluid, similar to the gas bearing described above. In a fluid bearing, some fluid vapor will typically remain and its pressure is generally related to the vapor pressure of the fluid. Vapor pump-out slots may be provided to reduce the vapor pressure within the air bearing and the sealing sheet structure can reduce the vapor pressure in the vacuum system to a tolerable level.